MySQL: ใช้ตัวแปร variable

((accumulatedScore / 100) * 50) +
((midtermScore / 100) * 20) +
((finalScore / 100) * 30) AS score

-- phpMyAdmin SQL Dump
-- version 4.7.0
-- https://www.phpmyadmin.net/
--
-- Host: 127.0.0.1
-- Generation Time: Oct 12, 2017 at 07:46 PM
-- Server version: 10.1.25-MariaDB
-- PHP Version: 7.1.7
 
SET SQL_MODE = "NO_AUTO_VALUE_ON_ZERO";
SET AUTOCOMMIT = 0;
START TRANSACTION;
SET time_zone = "+00:00";
 
--
-- Database: `snippets`
--
 
-- --------------------------------------------------------
 
--
-- Table structure for table `schoolReport`
--
 
CREATE TABLE `schoolreport` (
  `studen_id` int(11) NOT NULL,
  `course_id` int(11) NOT NULL,
  `accumulatedScore` int(11) NOT NULL,
  `midtermScore` int(11) NOT NULL,
  `finalScore` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
 
--
-- Dumping data for table `schoolReport`
--
 
INSERT INTO `schoolReport` (`studen_id`, `course_id`, `accumulatedScore`, `midtermScore`, `finalScore`) VALUES
(1, 1, 55, 76, 74),
(1, 2, 74, 74, 74),
(1, 3, 43, 76, 75),
(1, 4, 47, 45, 57),
(1, 5, 71, 45, 72),
(2, 1, 45, 85, 74),
(2, 2, 65, 47, 47),
(2, 3, 56, 85, 20),
(2, 4, 37, 75, 42),
(2, 5, 65, 35, 74);
 
--
-- Indexes for dumped tables
--
 
--
-- Indexes for table `schoolReport`
--
ALTER TABLE `schoolReport`
  ADD UNIQUE KEY `studen_id` (`studen_id`,`course_id`);
COMMIT;

SELECT
   studen_id,
   course_id,
   accumulatedScore,
   midtermScore,
   finalScore,
   ((accumulatedScore / 100) * 50) +
((midtermScore / 100) * 20) +
((finalScore / 100) * 30) AS score,
   CASE
      WHEN
         ((accumulatedScore / 100) * 50) +
((midtermScore / 100) * 20) +
((finalScore / 100) * 30) < 50
      THEN
         'F'
      WHEN
         ((accumulatedScore / 100) * 50) +
((midtermScore / 100) * 20) +
((finalScore / 100) * 30) <= 60
      THEN
        'D'
      WHEN
         ((accumulatedScore / 100) * 50) +
((midtermScore / 100) * 20) +
((finalScore / 100) * 30) <= 70
      THEN
         'C'
      WHEN
         ((accumulatedScore / 100) * 50) +
((midtermScore / 100) * 20) +
((finalScore / 100) * 30) <= 80
      THEN
         'B'
      else
         'A'
   END
   AS grade
FROM
   schoolReport

      WHEN
         ((accumulatedScore / 100) * 50) +
((midtermScore / 100) * 20) +
((midtermScore / 100) * 30) <= 70
      THEN
         'C'

SELECT
   studen_id,
   course_id,
   accumulatedScore,
   midtermScore,
   finalScore,
   @score := ((accumulatedScore / 100) * 50) +
((midtermScore / 100) * 20) +
((finalScore / 100) * 30) AS score,
   CASE
      WHEN
         @score < 50
      THEN
         'F'
      WHEN
         @score <= 60
      THEN
        'D'
      WHEN
         @score <= 70
      THEN
         'C'
      WHEN
         @score <= 80
      THEN
         'B'
      else
         'A'
   END
   AS grade
FROM
   schoolReport